Oxidation testing is an important part of assessing the potential stability of a lubricant for use in most lubricating applications including air compressors and gear oils. The high volumes of air and high temperatures experienced by a lubricant in an air compressor can have a large affect on the lubricant's oxidative stability. Assessing the stability of a lubricant in oxidation tests are methods by which a formulator can determine the potential stability of a air compressor lubricant in service.
An oxidation test has been developed to determine the length of time it takes for a lubricant to degrade from oxidation or break to a catastrophic increase in viscosity. This method is used to evaluate mineral and synthetic lubricants, with or without additives. The evaluation is based on the resistance of the lubricant to oxidation by air under specified conditions as measured by the changes in viscosity.
The sample is placed in a oxidation cell together with various organometallic catalysts that are dissolved in solution and then placed into the test cell. The cell and its contents are placed in a heating block maintained at a specified temperature, and a measured volume of dried air is bubbled through the test cell held at a pressure ranging from 0-100 psig for the duration of the test, with a air flow rate up to 250 cc/min. A constant temperature block, equipped with an electric heater and thermostatic control capable of maintaining the temperature within ±1° F. (0.5° C.) in the range of 200° F. (93° C.) to 450° F. (232° C.) is used to maintain the specified temperature.
Periodically the test cell is sampled for viscosity, until the oil has oxidized, identified by a rapid increase in oil viscosity. The oil condition is examined by measuring its Kinematic Viscosity at 100° C. Comparisons can then be made to the original Kinematic Viscosity at 100° C. of the oil. Good performance in this test is evidenced by little or no viscosity increase at end of test.
Hydrolytic stability is another important property for determining the stability of lubricants in the presence of moisture. For example, air compressor lubricants are exposed to high moisture levels as a result of normal compressor operations or condensation as the equipment cools after a shut down. Lubricants that are degraded by moisture can lead to increased oil oxidation and decreased lubrication properties which can lead to increased equipment corrosion and damage. ASTM D 2619 is an industry standard for testing hydrolytic stability in lubricants.
The air release properties of a lubricant is a key feature of determining how effective the lubricant is at releasing air from the lubricant after compression. Lubricants with poor air release properties may exhibit increased foaming and poor lubrication properties. All lubricating oil systems contain some air. It can be found in four phases: free air, dissolved air, entrained air and foam. Free air is trapped in a system, such as an air pocket in a hydraulic line. Dissolved air is in solution with the oil and is not visible to the naked eye. Foam is a collection of closely packed bubbles surrounded by thin films of oil that collect on the surface of the oil.
Air entrainment is a small amount of air in the form of extremely small bubbles (generally less than 1.0 mm in diameter) dispersed throughout the bulk of the oil. Agitation of lubricating oil with air in equipment, such as bearings, couplings, gears, pumps, and oil return lines, may produce a dispersion of finely divided air bubbles in the oil. If the residence time in the reservoir is too short to allow the air bubbles to rise to the oil surface, a mixture of air and oil will circulate through the lubricating oil system. This may result in an inability to maintain oil pressure (particularly with centrifugal pumps), incomplete oil films in bearings and gears, and poor hydraulic system performance or failure. Air entrainment is treated differently than foam, and is most often a completely separate problem. A partial list of potential effects of air entrainment include: pump cavitation, spongy, erratic operation of hydraulics, loss of precision control; vibrations, oil oxidation, component wear due to reduced lubricant viscosity, equipment shut down when low oil pressure switches trip, “micro-dieseling” due to ignition of the bubble sheath at the high temperatures generated by compressed air bubbles, safety problems in turbines if overspeed devices do not react quickly enough, and loss of head pressure in centrifugal pumps.
Antifoamants, including silicone additives help produce smaller bubbles in the bulk of the oil. In stagnant systems, the combination of smaller bubbles and greater sheath density can cause serious air entrainment problems. Turbine oil systems with quiescent reservoirs of several thousand gallons may have air entrainment problems with as little as a half a part per million silicone.
Casual exposure to silicone can have a significant effect on the lubricant. There are reports of air entrainment resulting from oil passing through hoses that had been formed on a silicone-coated mandrel. In one instance, in a turbine application, all sources of air were removed, and the system was carefully evaluated, component by component, to check for sources of contamination. After an exhaustive search, the culprit was found to be a silicone coating on electrical cables that were immersed in oil. Other known causes of entrainment problems include contaminants, overadditizing and reservoir design.
One method widely used to test air release properties of petroleum oils is ASTM D 3427-03. This test method measures the time for the entrained air content to fall to the relatively low value of 0.2% volume under a standardized set of test conditions and hence permits the comparison of the ability of oils to separate entrained air under conditions where a separation time is available. The significance of this test method has not been fully established. However, entrained air can cause sponginess and lack of sensitivity of the control of turbine and hydraulic systems. This test may not be suitable for ranking oils in applications where residence times are short and gas contents are high.
In the ASTM D3427 method, compressed air is blown through the test oil, which has been heated to a temperature of 25°, 50°, or 75° C. After the air flow is stopped, the time required for the air entrained in the oil to reduce in volume to 0.2% is recorded as the air release time.
Most solutions to the air entrainment problem have been to redesign the reservoir or choose additives not likely to cause aeration issues. There is a need to create a new formulations utilizing novel base stock combinations that have optimized improved oxidation, hydrolytic stability and air release properties while maintaining other favorable lubricating properties. Accordingly, this invention satisfies that need.